It is known to design drive shafts such that their critical speeds (the speeds at which they resonate) do not coincide with the speeds of operation of the equipment being driven. This helps avoid torsional excitation of the drive shaft. It is not always possible to so design a drive shaft. Further, torsional excitation of a drive shaft may occur due to operation of other equipment not driven by the drive shaft but in the same vicinity. Torsional excitation of a drive shaft may also occur due to operation of equipment connected to the equipment being driven, e.g. connected by an electrical circuit. This is especially so since the advent of high powered electronic control equipment utilizing thyristors.